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Reference Document - MCU12/0133.04
Bligh Tanner Pty Ltd + Directors
ABN 32 061 537 666 Paul Easingwood
[email protected] Alan Hoban
blightanner.com.au Nathan Scott
Simon Kochanek
+ Locations
Brisbane
Sydney
REFERENCE DOCUMENT
MCU12/0133.04
TECHNCIAL MEMORANDUM NOOSA COUNCIL
10 MARCH 2025
To: Carol Dowd <[email protected]>
Cc:
From: Vancy Li
Date: 19 February 2025
Pages: 5
RE: Noosa FLC Extension to Existing Gymnasium and Additional Pavements
Ref. No. 2025.0108
______
This technical memo outlines a comparison between the proposed development changes and the original
site-specific Stormwater Management Plan (dated 08/08/2012, as prepared by Bligh Tanner).This memo is
intended as an addendum to the previous approved report.
Summary of Development Changes
The proposed minor extension to the existing Gymnasium at the Noosa Flexible Learning Centre (FLC)
includes an increase in Gross Floor Area (GFA) by 17 m², with the extension positioned beneath the existing
roofline. Additionally, minor pavement works are planned, adding 77 m² of impervious surface, comprising a
new pathway and extension of the slab for the Physical Education (PE) Court.
The primary objective of these modifications is to enhance the facilities at the Noosa FLC, ensuring improved
functionality while maintaining the existing staff and student capacity. The letter also considers previous
council-approved works that have been constructed following the original stormwater management plan,
which did not necessitate further engineering review or assessment in relation to stormwater management.
Table 1 Summary of Changes
ORIGINAL ADDITIONAL PROPOSED
DEVELOPMENT APPROVED DEVELOPMENT
APPLICATION WORKS
Additional Slab on ground area - 100 77
Total Slab on Ground Area 177 277 354
Percentage of Slab on ground of the site 3.2% 5.0% 6.3%
Additional Roof Area - 144 0
Roof/Covered Area 1404 1548 1548
Percentage of Covered area of the site 25.1% 27.6% 27.6%
Area Free of Structure 3808 3664 3587
Percentage of Area Free of Structure of the site 68.00% 65.43% 64.05%
The changes of hardstand from the proposed works increase the percentage of slab-on ground on site only
by 1.3%, which still falls within 10% of the overall site area. And no extra roof area has been proposed as part
of the proposal.
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Noosa FLC Extension to Existing Gymnasium and Additional Pavements
19 February 2025 Ref. No. 2025.0108
Alignment of Proposed Changes with the Existing Stormwater Management Strategies
The previous stormwater management strategy incorporates several key features to ensure minimal impact
on site permeability, runoff, and pollutant generation. The proposed development has been assessed against
these features as below.
Table 2 Alignment of Proposed Changes with the Existing Stormwater Management Strategy
KEY FEATURE DETAILS COMMENTS ON PROPOSED
DEVELOPMENT
Ground-Level Permeability - At least 90% of the site will - Despite a slight increase in slab-
remain permeable to support on-ground area, the site remains
natural infiltration. - 3% of the 90% permeable, maintaining
site (177 m²) will have slab-on- natural infiltration.
ground construction (Music
building for soundproofing, and
part of the Create building due to
raised ground levels). - Most
structures will be on raised piers
to preserve infiltration
characteristics and protect
shallow tree roots.
- 7% of the site will be car
parking, using permeable
pavement to reduce runoff.
Minimal Structural Footprint - 68% of the site will remain free - 64% of the site will remain free
of buildings or car parks, of building or car parks
minimizing changes to pollutant
filtration and infiltration
properties.
- Includes retention of existing
vegetation and revegetation with
native species.
Roof Runoff Management -25% of the site will be covered - The increase in roof area is only
by roofs. 2.6% compared to previous
- 60% of roof areas will drain into approved works.
rainwater tanks for non-potable - No additional roof area is
water use. proposed for this application.
- Rainwater tanks and infiltration
- 60% of captured water will be
trenches remain functional for
reused, with overflow directed to
water reuse and natural
infiltration trenches.
infiltration.
- Some roof areas will drain
directly into infiltration trenches
instead of rainwater tanks, due to
the site’s low non-potable water
demand.
- The mean annual volume of
surface runoff leaving the site will
be 20% lower than existing levels
due to rainwater harvesting and
infiltration.
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Noosa FLC Extension to Existing Gymnasium and Additional Pavements
19 February 2025 Ref. No. 2025.0108
KEY FEATURE DETAILS COMMENTS ON PROPOSED
DEVELOPMENT
Runoff Treatment & - Site runoff will enter the The existing water tank and
Environmental Protection stormwater drainage system infiltration system will be
along Girraween Court. retained. A new MUSIC model,
- The drainage system flows incorporating both previous and
through an existing proposed works, has been
bioretention/infiltration system provided. Refer to ‘MUSIC
before reaching Noosa National Modelling’ Section for details.
Park, Girraween Nature Reserve,
and Burgess Creek.
- While not specifically designed
for the site, the bioretention
system provides additional
environmental protection.
- No significant pollutant loads
are expected due to the site’s
intended use.
Peak Flow & Flood Management - The small increase in The peak flows for stormwater run-
impervious areas will lead to a off from the site is likely to
minimal increase in peak flows increase marginally. However, the
during major storms. run-off will be contained within the
- Approximately 1 m³ of flood road reserve of Girraween Court
detention will be required and which ultimately discharges into
can be easily accommodated in Burgess Creek at the end of the
the final site design through Girraween Court. It is unlikely to
infiltration systems, landscape cause any worsening to third party
depressions, or additional property. In addition, on-site
rainwater tank airspace. detention is likely to cause a
coincidence with external
hydrograph peaks at confluence
locations.
Therefore, no on-site detention is
deemed required or desirable for
the proposed works.
Overland Flow Path - Overland flow paths will remain The overland flow path is expected
Considerations consistent with existing to remain unchanged with the
conditions. minimal increase to hardstand.
- During major storm events,
water will sheet flow across the
southwestern boundary onto
Girraween Court, then west
towards Burgess Creek.
In summary, the proposed works align with the original stormwater management strategies.
Erosion and Sediment Control
This site is considered low risk based on the following erosion hazard assessment:
• Predicted Annual Soil loss (tonnes) = A x B x T
• Where A = R.K.LS.C.P (as per the Revised Universal Soil Loss Equation).
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19 February 2025 Ref. No. 2025.0108
Table 3 Values used in erosion hazard estimation
FACTOR DESCRIPTION ADOPTED VALUE
R Rainfall erosivity factor 5700 (i 2y 6hr = 15.8 mm/hr)
K Soil erodibility factor 0.04 (loam)
LS Topographic factor 0.2 (100 m @ 1%)
C Cover and management 1 (no cover)
P Practice 1.3 (no management)
B Area of disturbance 0.1 ha*
T Predicted duration 0.25 yrs
*The area of disturbance should be less than 0.1 hectare as the proposed hardstand is only 77m.
• Predicted unmitigated soil loss = 59 t/ha/yr x 0.1 ha x 0.25 yrs = 1.5 tonnes
As this is significantly less than 150 tonnes, this development is considered low risk. Erosion and sediment
control measures should be implemented in accordance with the International Erosion Control Association
(IECA) Best Management Practices (2016), the latest version of the IECA guidelines, Council requirements
and the SPP during construction.
MUSIC Modelling
The MUSIC Model has been configured to represent the additional approved hardstand and the proposed
slab on-ground areas.
Figure 1 MUSIC Model Schematic
The comparison of the runoff volumes and pollutant loads have been compared to as below:
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19 February 2025 Ref. No. 2025.0108
Figure 2 Comparison of Runoff Volumes and Pollutant Loads
Table 4 Comparison of runoff volumes and pollutant loads
EXISTING
DEVELOPMENT
COMMERCIAL
CONVENTIONAL
PRACTICE* BIORETENTION
COMMERCIAL WITH BEST
CONVENTIONAL
NOOSA FLEXI SCHOOL
TANKS
NOOSA FLEXI SCHOOL WITH
TANKS + INFILTRATION
NOOSA FLEXI SCHOOL WITH
+ADDITIONAL HARDSTAND
TANKS + INFILTRATION
NOOSA FLEXI SCHOOL WITH
Flow (ML/yr) 2.9 7.8 7.6 4.6 3.9 2.0 2.9
Total Suspended Solids 160 1300 270 330 300 150 228
(kg/yr)
Total Phosphorus (kg/yr) 0.19 3.0 1.2 0.8 0.65 0.22 0.27
Total Nitrogen (kg/yr) 2.1 25 14 7.3 5.5 1.9 2.5
* sized to reduce pollutant loads by 80% (TSS) 60% (TP) and 45% (TN)
In summary, the mitigated pollutant loads are close to existing level and well below a conventional ‘best-
practice’ approach.
Conclusion
The proposed works are consistent with the original Stormwater Management Plan submitted for
MUC12/0133. Additionally, the erosion hazard assessment has been classified as ‘low risk,’ indicating
minimal environmental impact. The proposed works completed to date and proposed as part of this current
application result in pollutant loads in keeping with the optimum results modelled in the original Stormwater
Report and a significant improvement over more conventional approach.
Yours faithfully,
Vancy Li
Civil and Water Engineer
BLIGH TANNER PTY LTD